In electrostatography, an electrostatic toner, which is a developer material for electrophotography is used for developing a visible image from a latent image which has been created by application of a charge pattern on the surface of a photoreceptor. An electrostatic toner is a finely divided chargeable powder which may comprise a resin having dispersed therein a coloring agent such as a pigment or carbon black. Electrostatic toners may be categorized into dry two-component toners which are used with a carrier such as iron powder or glass particles; wet toners which comprise a dispersion system in an organic solvent such as isoparafin; and dry one-component toners wherein a magnetic finely divided powder is dispersed.
The image which has been developed on the photoreceptor with the electrostatic toner is transferred onto a paper from the photoreceptor, and on the paper, the image is fixed by means of heat or a vaporized solvent. In the case of an image which has been directly developed on a paper having disposed thereon a sensitizing layer, the image is fixed by means of heat or a vaporized solvent with no further transfer. Among various methods of fixing, fixing of an image using a heated roller is an advantageous method since it is a contact fusing at a high thermal efficiency which may enable a reliable fixing of the image even by using a heat source at a relatively low temperature, and since such a method is adequate for a high-speed reprography.
However, when a conventional electrostatic toner is used and the image is fixed onto a paper by bringing a heated body such as a heated roller into contact with the image, a phenomenon called offset is likely to be induced. When such offset is induced, a part of the image becomes adhered to the heated body and the thus adhered image is then transferred onto the part of the paper where subsequent portion of the image is transferred. In particular, in a high-speed reprography wherein the temperature of the heated body must be elevated to increase fixing efficiency and fixing rate, such offset phenomenon is more likely to be induced. In view of such a situation, in the case of fixing an image created by a one-component electrostatic toner using a heated roller, the roller surface is often impregnated with or fed with a silicone oil to thereby avoid such an offset phenomenon from being induced. Such a countermeasure, however, may induce another problem that the roller may become susceptible for contamination.
For a binder, which is one of the main components in the electrostatic toner, various thermoplastic resins have been employed. Among such thermoplastic resins which have been employed for the binder, low-molecular weight styrene-(meth)acrylate copolymers have the merits that they are highly chargeable; that they exhibit an excellent fixing owing to their adequate softening point (approx. 100.degree. C.); that cleaning of the photoreceptor is easy and the photoreceptor is less susceptible to contamination; that they are less hygroscopic; that they may be sufficiently mixed with carbon black which is the colorant; and that they may be readily pulverized. However, such conventional electrostatic toners prepared by using a low-molecular weight styrene-(meth)acrylate copolymer may still suffer from the offset phenomenon when they are used in the high-speed reprography.
In order to obviate the above-described problems, inclusion in the electrostatic toner of a polyolefin wax as a releasing agent has been proposed (Japanese Patent Publication Nos. 52(1977)-3304, 52(1977)-3305, 57(1982)-52574, 58(1983)-58664, and Japanese Patent Application Kokai No. 58(1983)-59455).
Even when the techniques described in the above-mentioned Japanese Patent Publication No. 52(1977)-3304 and the like are used, various problems are still induced since the properties of the polyolefin wax as a releasing agent are not fully exerted. For example, when a polyolefin wax of a relatively low molecular weight is used for the releasing agent, release properties upon thermal fixing and resistance to low-temperature offset phenomenon of the resulting toner may be improved while fixing to the paper of the fixed image would become poor due to the low mechanical strength of the wax itself. On the contrary, use of a high-molecular weight wax would result in an improved fixing to the paper of the fixed image and an improved resistance to high-temperature offset of the resulting toner. The resistance to low-temperature offset, however, would become poor due to an increase in softening point.